This invention relates to improved nematic liquid crystal compositions and to display devices including these compositions.
Nematic liquid crystal compositions are used in electro-optical display devices, as described in U.S. Pat. No. 3,222,485 to Williams. Such devices normally comprise a thin layer of the liquid crystal composition sandwiched between two planar elements. One of the elements is transparent while the other is either transparent, reflective or absorptive, depending on the mode of operation of the device. Conductors are applied to a surface of each of the elements for applying an electric field to selected areas of the liquid crystal layer. When an electric field above a threshold value is applied to the liquid crystal layer, the layer appears to change the intensity of transmitted light in the region of the applied field.
When a negative dielectric anisotropic nematic material is employed as the liquid crystal composition, the apparent change in light intensity is due to a change in the light scattering properties of the composition under the electric field; and when a positive dielectric anisotropic nematic material is employed, the change is due to a change in the ability of the composition to rotate polarized light.
Liquid crystal compositions have two transition temperatures. One is at the transition between the crystalline solid state and the liquid crystal state. The other is at the transition between the liquid crystal state and the isotropic fluid state. Since the composition must be in the liquid crystal state for operation of the display device in which it is employed, it is desirable that the composition be in the liquid crystal state over a broad temperature range (referred to as the nematic range) having its center at room temperature so that the device will operate at varying ambient temperatures.
Other desirable properties of liquid crystal compositions are a high contrast ratio (the ratio of light scattering when an electric field is applied to that when no field is applied) and quick response (rise and decay) times.
As described in U.S. Pat. 3,656,834 to Haller et al., it is desirable to add to a liquid crystal composition a material which improves the homeotropic alignment of the molecules in the composition. Such materials are commonly referred to in the art as "dopants." In addition to improving homeotropic alignment, dopants provide ions which enable negative dielectric anisotropic nematic materials to enter the dynamic scattering mode when subjected to electric fields above threshold voltage. While sufficient dopant must be added so that the composition is in the dynamic scattering mode at desired voltage and frequency, excessive dopant is undesirable because it causes too much current flow, which, in addition to consuming electricity, causes degradation of the liquid crystal molecules.
It is an object of this invention to provide a liquid crystal composition having a broad nematic range with its center at room temperature, and also having other improved properties, including high contrast ratio, quick response times.